The invention relates to a magnetic resonance imaging (MRI) system comprising an examination volume, a main magnet system for generating a main magnetic field in the examination volume, a gradient magnet system for generating gradients of the main magnetic field, and a control system for compensating disturbances of the magnetic field caused by mechanical vibrations of the MRI system.
In general, the main portions of an MRI system, like the main magnet system and the gradient magnet system, are subject to mechanical vibrations during operation. These vibrations are caused by environmental disturbances, such as floor vibrations or movements of the patient in the MRI system, and also by internal disturbances, such as the altering Lorentz forces which are exerted on the electric coils of the gradient magnet system as a result of the interaction between the main magnetic field and the altering electric currents in said coils. The vibrations lead to unwanted time-dependent and periodical disturbances of the magnetic field in the examination volume. As a result of said disturbances of the magnetic field, the spatial definition of the magnetic field in the examination volume is not sufficiently accurate, so that artifacts and other distortions are introduced into the reconstructed image of the subject to be examined in the examination volume.
An MRI system of the kind mentioned in the opening paragraph is known from U.S. Pat. No. 6,396,268. The known MRI system is of the so-called open type, in which the main magnet system is accommodated in a magnet top half and in a magnet bottom half between which the examination volume is present. The magnet top half and the magnet bottom half are held at a pre-determined distance from each other by means of a vertical support post. The control system of the known MRI system comprises a feed-back control loop having a sensor, which is mounted on the support post and measures disturbances of the magnetic field. In the known MRI system said disturbances are mainly caused by mutual position changes of the magnet top half and the magnet bottom half. The sensor generates a magnetic field change signal which is supplied to a controller. The controller converts the magnetic field change signal into a compensation signal which corresponds to a desired compensation for the measured disturbances of the magnetic field. In a first embodiment, the known MRI system comprises a number of compensation coils which actively generate a compensating magnetic field for the measured disturbances of the magnetic field, the compensation signal corresponding to a desired electric current in the compensating coils. In a second embodiment, the compensation signal is supplied to a transceiver of the MRI system which controls the RF transceiver coils of the MRI system. For a desired selected position in the examination volume, the transceiver compensates the frequency of the RF signals transmitted by the RF transceiver coils for the measured disturbances of the magnetic field, so that the position actually selected is not influenced by the disturbances of the magnetic field. In a third embodiment, a compensating magnetic field is generated by the gradient magnet system. In this embodiment the compensation signal corresponds to a desired additional compensating electric current which is supplied to the gradient magnet system, the compensation signal being added to the driving signal of the coils of the gradient magnet system.
A disadvantage of the known MRI system is that the compensation of the disturbances of the magnetic field achieved by means of the control system is not sufficiently accurate. A first reason is that the sensor does not measure the disturbances of the magnetic field in the examination volume, i.e. at the location where the disturbances of the magnetic field should be compensated. It is not practical or even impossible to arrange the sensor in the examination volume. A second reason is that the magnetic field at the location of the sensor is influenced by the gradient magnet system, so that the sensor does not solely measure the disturbances of the main magnetic field.